The organic compounds derived from biomass (e.g. wood) have long been used as fuel, mainly for heating. Owing to its relatively low calorific property, this scarcely profitable use has been superseded more recently by another conversion of better advantage, namely gasification which consists of converting organic compounds to useable gases, particularly as gas fuel for example in fuel cells or as basic gas constituents for the synthesis of chemical compounds.
The gasification of compounds derived from biomass typically entails steam gasification in which the compounds it is desired to convert are placed in the presence of steam and a suitable catalyst, at relatively high temperatures. Following a complex thermo-chemical process this results in the conversion of organic compounds to a gas mixture (called syngas) which chiefly contains hydrogen, carbon monoxide and/or carbon dioxide, and methane. Among the thermo-chemical conversion processes currently used, fluidized bed gasification with steam allows the obtaining of optimized yields of gas products (hydrogen in particular) by means of the high heating temperatures involved, whilst proposing advantageous residence times and a reduction in the quantities of residues produced with, in addition, a high calorific power of the gas produced (higher than with gasification processes in air which lead to nitrogen-diluted gases).
A recurrent problem which arises with these gas mixtures derived from steam gasification processes of organic compounds is that the steam gasification processes of organic compounds are accompanied by the formation of undesirable by-products likely to pollute the syngases. In particular, notably on account of the high temperatures involved, the steam gasification reactions often give rise to tars.
The term <<tar>> in the meaning of the present description is meant to designate an aromatic compound having a higher molecular weight than benzene (for example toluene or the naphtalenes), or else a mixture comprising said aromatic compounds. The tars which are obtained with a steam gasification process typically include compounds comprising 1 to 5 aromatic cores, whether or not oxygenated.
The presence of the said tars in the gases derived from steam gasification makes these gases unfit for some industrial applications, for example in gas turbines or fuel cells. The tars effectively tend to condense into complex structures liable to clog these devices. This disadvantage is a major obstacle to the economic viability of biomass steam gasification processes.
More generally, the generation of tars during steam gasification raises another problem, namely that the tars produced are liable to poison the catalyst used to carry out gasification, since the tars tend to deposit on the surface of the catalyst and consequently to cause its gradual de-activation.
Different methods have been proposed for removing these tars, either downstream of the gasification process (so-called secondary removal methods) or directly in the reactor in which these tars are formed (so-called primary methods which allow the inhibiting of tar formation and/or the conversion of the formed tars within the reactor) which do not always prove to be satisfactory.
Among the secondary methods, chemical methods have been envisaged (in particular thermal cracking) or mechanical methods (filters, cyclones, centrifuges, scrubbers) which although efficient in reducing the tar content of gases derived from biomass steam gasification processes, prove to be costly particularly in terms of energy consumption (for cracking in particular) or in treatment terms (with regard to cracking filters or scrubbers in particular).
Concerning the primary methods, in an attempt to remove tars or to inhibit their formation at source, it has been envisaged to add different types of catalysts to steam gasification reactors, which in practice have proved to be more or less efficient. The proposed catalysts are generally intended to achieve catalytic reduction of tars using reforming reactions consisting of converting the tars in the presence of water and/or CO2 to a gas mixture notably comprising hydrogen, CO and/or CO2, typically in accordance with one and/or the other of the following reactions (in which CnHm represents a tar):CnHm+nH2O→nCO+(n+m/2)H2 CnHm+nCO2→2nCO+m/2H2 Within this context it has been proposed, as catalyst, to use sand, dolomites and calcined magnesites, or even olivines and zeolites, and catalysts based on metal compounds (supported nickel oxides in particular). The results obtained are more or less conclusive with catalytic efficacies possibly varying to a large extent including for catalysts with close compositions. In this respect, it is to be noted that the reactions which take place within the gasification reactor are ill understood and that the choice of suitable catalyst is therefore made empirically. The mechanisms involved are insufficiently understood at the present time making it impossible to predict beforehand whether a catalyst will or will not be efficient before it is actually tested.
Among the proposed catalysts, the dolomites (magnesium and calcium carbonates) need to be calcined to oxide form, which leads to loss of specific surface area and brittleness which generate phenomena of catalyst attrition (gradual crumbling of the catalyst) and lead to the undesired formation of fine particles in the gas effluent which makes the use of dolomites difficult, in particular in fluidized bed reactors notably at industrial level.
Olivines (mixed silicates of magnesium and iron typically meeting the formula (Mg1-xFex)2SiO4 where x is a nonzero number and generally of the order of 0.1) are more resistant to attrition. Although they have some activity in the destruction of tars (similar to that of calcined dolomites) they nevertheless prove to be insufficiently efficient for the envisaging of their use on an industrial scale.
Concerning the catalysts based on metal compounds, different catalysts have been tested in reactions of biomass steam gasification type. In this respect, nickel catalysts have especially been developed of which some have proved to allow relatively efficient tar reforming, in particular at applied temperatures of higher than 740° C., with substantial removing of tars, to a greater extent than with the above-mentioned olivines and calcined dolomites. For example nickel-bases catalysts supported on alumina have particularly been described (in Ind. Eng. Chem. Res., vol. 36, p 1335 (1997), for example) or more recently, in application FR 2 809 030, catalysts which comprise a nickel-based active phase deposited on an olivine substrate which have given excellent results for the steam reforming of tars.
Nonetheless, despite these advantages, nickel-based catalysts have the drawback of being subject to phenomena of attrition, in particular when used in fluidized bed reactors. In addition to a loss of catalytic efficacy over time, these phenomena of attrition lead to non-negligible pollution of the gases produced by the nickel, which is unacceptable at industrial level having regard to the toxicity of nickel.
In addition, nickel catalysts tend to become deactivated when in contact with compounds such as sulfur-containing compounds which are often obtained during the steam gasification of biomass. The nickel in oxidation state 0 which forms on the surface of nickel catalysts also proves to be a coke precursor likely to lead to the formation of carbon aggregates (cokes) which may poison the catalyst. Also, nickel-based catalysts require recycling after their use, which proves to be particularly costly.